Annealing (sobi)

Strain Hardening and Annealing

Chapter 7 – 4th EditionChapter 8 – 5th Edition

Strain Hardening in MetalsWhen a piece of metal is deformed, the dislocations run into each otherThis traffic jam increases the material’s strengthDeforming a piece of metal also actually increases the number of dislocationsThis increases the strength too!!

Tensile Test

You can understand this better by relating it to the results of the tensile test.

John D Russ – Materials Science – A Multimedia Approach

Try it!!

Strain harden a piece of copper tubing.

Effects of Strain Hardening

Yield Strength goes upTensile Strength goes upDuctility goes down The material becomes brittle

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Strain Hardening Coefficient

Strain Hardening Coefficient is a measure of how much the metal can be strengthened by strain hardeningIt needs to have some ductility to be strain hardened.

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n is the slope of the PLASTIC portion of the curve, when graphed on a logarithmic scale

Log plot of the plastic portion of a tensile test

Log

True

Stre

ss, σ

Log True Strain, ε0.001 0.010 0.100 1.0

K

Slope=n

Log(σ) = Log(K) + n Log(ε)

Effect of Crystal Structure

HCP metals are already brittle Little strain hardening is possible Strain hardening coefficient around 0.05

BCC metals are less brittle than HCP Some strain hardening is possible N around 0.15

FCC metals are ductile Strain hardening is easy N around 0.5

Frank-Read Source

Strain hardening actually increases the strength of a material PAST its original tensile strengthWhy?Additional dislocations are formed as dislocations run into point defects

Yield Point

Tensile Strength

Strain Hardening – The effect of dislocation generation

From “Materials Science – A Multimedia Approach”, by John Russ

Copper 30% zinc alloy

Frank-Read Source

Point Defect Point Defect

Dislocation

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_5/backbone/r5_3_2.html

Frank-Read Source

Frank-Read Source

Before deformation a typical dislocation density is about 106 cm of dislocation per cm3 of metalAfter strain hardening it may increase to as much as 1012 cm per cm3 of metal

Strain Hardening in Polymers

When you pull on a polymer, the chains line upVan der Waal bonds form between the chainsThe polymer becomes stronger

Try it with a 6-pack ring!!The mechanism for strain hardening in plastics is different from the mechanism in metals

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http://www.indiamart.com/amd-metplast/pcat-gifs/products-small/bottel-preform.jpg

PET Bottles and Preforms

Strain Hardening in Ceramics?

Ceramics are already brittle – so strain hardening is not usually possibleCeramics break because of flaws – the mechanism of deformation is differentAnnealing ceramics causes grain growth May or may not be bad

Back to MetalsCold Work

There is only a certain amount you can deform a material before it breaksCold work is strain hardening measured in % - The percent change in cross sectional area of the materialDifferent materials have varying % allowable cold work

Wire Drawing

d0

Initial diameter

Final diameter

d

Initial Cross Sectional Area - Final Cross Sectional Area% Cold Work =

Initial Cross Sectional Area

F

Rod

Deformation of a rod (or a piece of wire)Initial cross sectional area minus Final cross sectional areaOver the initial cross sectional area

100*0

0

−A

AA

−=

−=

Π

Π−Π=20

220

20

220

20

220

*

**%

d

dd

r

rr

r

rrCW

All times 100 of course

Copper is often drawn into wire

Copper rod feeding into drawing machine

Plate Rolling

h0

Initial thickness

h

Final thickness

Initial Cross Sectional Area - Final Cross Sectional Area% Cold Work =

Initial Cross Sectional Area

Cold Rolling

Metal is often rolled into sheets from thicker stockThe width of the sheet is usually kept the same, and only the thickness varies

( )oh

hh

Wh

hhW

A

AACW

−=−=

−= 0

0

0

0

0%

Cold Rolled Steel

Cold Work

What if you want to deform the sample more than is “possible?For example, what if you want to draw a piece of wire, from a rod of copper?You can anneal the material, and “undo” the strain hardening

Annealing is a heat treatment

Problem

Propose a series of steps to reduce a rod of copper-zinc alloy from 1 “ diameter to .1”diameter.The maximum cold work allowable for this copper zinc alloy is 85%.You will have to draw the copper, then anneal it several times.

One solutionDraw the 1” rod to 0.5”

Anneal

Draw the 0.5” rod to 0.25”

Anneal

Draw the 0.25” rod to 0.125”

Anneal

2 2

2

1 0.50.75

1CW

−= =

2 2

2

0.5 0.250.75

0.5CW

−= =

2 2

2

0.25 0.1250.75

0.25CW

−= =

The maximum cold work is not exceeded

Final Step

Draw the 0.125” rod to 0.1”

2 2

2

0.125 0.10.36

0.125CW

−= =

The final cold work is 36%

Problem

What if you want a certain tensile strength in your final product?Look at one of the graphs of properties vs. cold work from the book.Make sure that your final cold work step is the right size to give you the properties you want.

From “Materials Science”, by John Russ

Cold Work is Anisotropic

When you deform a piece of metal you elongate the grain.Slip occurs only in the favored directionsYou strengthen the material in the direction it is deformed, but properties in the other directions do not change as much.

The Science and Engineering of Materials - Askeland

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©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

Cold Working Wire

When you draw wire, you strengthen in the longitudinal directionIt is not strengthened axiallyThis makes it easy to cut, but hard to break by pulling on it!!

Annealing

You can’t just haphazardly heat up a piece of metal to “undo” the strain hardeningIt’s a temperature dependent process

Annealing

RecoveryRecrystallizationGrain Growth

http://www.all-sourceheattreating.com/img/home_nologob.jpg

Recovery or Stress Relief

If you only add a small amount of thermal energy (heat it up a little) the dislocations rearrange themselves into networks to relieve residual stressesPolyganized subgrain structureDuctility is improvedStrength does not change

The Science and Engineering of Materials - Askeland

Three EBSD maps of the stored energy in an Al-Mg-Mn alloy after exposure to increasing recrystallization temperature. The volume fraction of recrystallized grains (light) increases with temperature for a given time.

Sometimes Residual Stresses are good

Shot PeeningTempered Glass Side and Rear Windows in Cars

http://abrasivefinishingcompany.com/images/shot_peening_1.jpg

Recrystallization

Add more heat, and new grains start to grow at the grain boundaries.The new grains have not been strain hardenedThe recrystallized metal is ductile and has low strength

The Science and Engineering of Materials - Askeland

Grain Growth

If you keep the metal hot too long, or heat it up too much, the grains become largeUsually not goodLow strengthAlso brittle

The Science and Engineering of Materials - Askeland

Check out the CD animations

Try the quiz on the CD!!On the next page explore how properties change during the annealing processThe whole process depends not only on the temperature, but on how long you keep the metal hot.

John Russ

Materials Science – A Multimedia Approach

Sometimes annealing happens by itself!!

Is cold working a good way to strengthen a metal used at high temperatures?What about a tungsten filament in a light bulb?

SEM of a tungsten filament

http://ion.asu.edu/descript_depth.htm

How hot is hot?

Most metals have a recrystallization temperature equal to about 40% of the melting point measured in Kelvin

mr TT 4.0=

For Example

If a metal melts at 1000K, it’s recrystallization temperature is approximately 400KIf the metal is exposed to temperatures above the recrystallization temperature while in service, the strengthening achieved with cold work will be eliminated

Factors Contributing to Recrystallization Temperature

Melting PointOriginal Grain SizeAmount of Cold WorkPure metals recrystallize at lower temperatures than alloysTime at temperature

Typical Recrystallization Temperatures

Metal Melting Temperature 0C

Recrystallization Temperature 0C

Sn 232 -4Pb 327 -4Zn 420 10Al 660 150Mg 650 200Ag 962 200Cu 1085 200Fe 1538 450Ni 1453 600Mo 2610 900

W 3410 1200

These metals recrystallize below room temperature – so cold work is not possible under normal conditions

The Science and Engineering of Materials - Askeland

What should you do if cold working isn’t applicable?

Try solid solution strengtheningTry hot working

http://www.bbc.co.uk/shropshire/content/image_galleries/friendship_through_iron_gallery.shtml

Hot Working

Shape the metal while it is hot. Above the recrystallization temperature

Blacksmiths use a combination of hot work and cold work.Can not fine tune the final properties this wayDimensional control is hardSurface finishes may be hard to produce

©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license.

During hot working, the elongated anisotropic grains immediately recrystal l ize. I f the hot-working temperature is properly controlled, the f inal hot-worked grain size can be very f ine.

Hot Rolling Steel

What happens when you weld a cold worked piece of metal?

Welding affects the surrounding material

An incomplete weld of a bike frame which failed. Apparent in the image is the bright weld material in the center, the surrounding lighter heat affected zone (HAZ), and dark outer unaffected base metal. Field of view is approximately 15 mm.

http://www.mse.mtu.edu/slides/slide_2.html

Used by Permission of Ruth Kramer

The following slides show the effect of cold working on various metals